The present invention relates to semiconductor manufacture, and particularly to systems and methods for supplying ultra-high-purity HF and hydrofluoric acid for semiconductor manufacture.
Background: Contamination in IC Manufacturing
Contamination is generally an overwhelmingly important concern in integrated circuit manufacturing. A large fraction of the steps in modern integrated circuit manufacturing are cleanup steps of one kind or another; such cleanup steps may need to remove organic contaminants, metallic contaminants, photoresist (or inorganic residues thereof), byproducts of etching, native oxides, etc.
As of 1995 the cost of a new front end (integrated circuit wafer fabrication facility) is typically more than a billion dollars ($1,000,000,000), and a large fraction of this cost is directed to measures for particulate control, cleanup, and contamination control.
One important source of contamnination is impurities in the process chemicals. Since the cleanups are so frequent and so critical, contamination due to cleanup chemistry is very undesirable.
The extreme purity levels required by semiconductor manufacturing are rare or unique among industrial processes. At such extreme purity levels, handling of chemicals is inherently undesirable (though of course it cannot be entirely avoided). Exposure of the ultrapure chemical to air (particularly in an environment where workers are also present) must be minimized. Such exposure risks introduction of particulates, and resulting contamination. Shipment of ultrapure chemicals in closed containers is still not ideal, since there is inherently a higher risk of contaminants at the manufacturer or at the user's site. Moreover, undetected contamination may damage an expensively large quantity of wafers.
Since many corrosive and/or toxic chemicals are commonly used in semiconductor processing, the reagent supply locations are commonly separated from the locations where front-end workers are present. Construction and maintenance of piping for ultra-high-purity gasses and liquids are well-understood in the semiconductor industry, so most gasses and liquids can be transported to wafer fabrication stations from anywhere in the same building (or even in the same site).
Wet Versus Dry Processing
One of the long-running technological shifts in semiconductor processing has been the changes (and attempted changes) between dry and wet processing. In dry processing, only gaseous or plasma-phase reactants come in contact with the wafer. In wet processing, a variety of liquid reagents are used for purposes such as etching silicon dioxide or removing native oxide layers, removing organic materials or trace organic contaminants, removing metals or trace organic contaminants, etching silicon nitride, etching silicon.
Plasma etching has many attractive capabilities, but it is not adequate for cleanup. There is simply no available chemistry to remove some of the most undesirable impurities, such as gold. Thus wet cleanup processes are essential to modern semiconductor processing, and are likely to remain so for the foreseeable future.
Plasma etching is performed with photoresist in place, and is not directly followed by high-temperature steps. Instead the resist is stripped, and a cleanup is then necessary.
The materials which the cleanup must remove may include: photoresist residues (organic polymers); sodium; Alkaline earths (e.g. calcium or magnesium); and heavy metals (e.g. gold). Many of these do not form volatile halides, so plasma etching cannot carry them away. Cleanups using wet chemistries are required.
The result of this is that purity of process chemicals at plasma etching is not as critical, since these steps are always followed by cleanup steps before high-temperature steps occur, and the cleanup steps can remove dangerous contaminants from the surface before high-temperature steps drive in these contaminants. However, purity of the liquid chemicals is much more critical, because the impingement rate at the semiconductor surface is typically a million times higher than in plasma etching, and because the liquid cleanup steps are directly followed by high-temperature steps.
However, wet processing has one major drawback, namely ionic contamination. Integrated circuit structures use only a few dopant species (boron, arsenic, phosphorus, and sometimes antimony) to form the required p-type and n-type doped regions. However, many other species are electrically active dopants, and are highly undesirable contaminants. Many of these contaminants can have deleterious effects, such as increased junction leakage, at concentrations well below 10.sup.13 cm.sup.-3. Moreover, some of the less desirable contaminants segregate into silicon, i.e. where silicon is in contact with an aqueous solution the equilibrium concentration of the contaminants will be higher in the silicon than in the solution. Moreover, some of the less desirable contaminants have very high diffusion coefficients, so that introduction of such dopants into any part of the silicon wafer will tend to allow these contaminants to diffuse throughout, including junction locations where these contaminants will cause leakage.
Thus all liquid solutions which will be used on a semiconductor wafer should preferably have extremely low levels of all metal ions. Preferably the concentration of all metals combined should be less than 300 ppt (parts per trillion), and less than 10 ppt for any one metal, and less would be better. Moreover, contamination by both anions and cations must also be controlled. (Some anions may have adverse effects, e.g. complexed metal ions may reduce to mobile metal atoms or ions in the silicon lattice.)
Front end facilities normally include on-site purification systems for preparation of high-purity water (referred to as "DI" water, i.e. deionized water). However, it is more difficult to obtain process chemicals in the purities needed.
Ammonia Purification
The present inventors have developed a method for preparing ultra-high-purity ammonia, in an on-site system located at the semiconductor wafer production site, by: drawing ammonia vapor from a liquid ammonia reservoir, passing the ammonia vapor through a microfiltration filter, and scrubbing the filtered vapor with high-pH purified water (preferably deionized water which has been allowed to equilibrate with the ammonia strearn). This discovery permitted conversion of commercial grade ammonia to ammonia of sufficiently high purity for high precision manufacturing without the need for conventional column distillation. The drawing of the ammonia vapor from the supply reservoir serves by itself as a single-stage distillation, eliminating nonvolatile and high-boiling impurities, such as alkali and alkaline earth metal oxides, carbonates and hydrides, transition metal halides and hydrides, and high-boiling hydrocarbons and halocarbons. The reactive volatile impurities that could be found in commercial grade ammonia, such as certain transition metal halides, Group III metal hydrides and halides, certain Group IV hydrides and halides, and halogens, previously thought to require distillation for removal, were discovered to be capable of removal by scrubbing to a degree which is adequate for high-precision operations. This is a very surprising discovery, since scrubber technology is traditionally used for the removal of macro-scale, rather than micro-scale, impurities.
Innovative Systems and Methods for Semiconductor Manufacturing with On-Site HF Purification
The present application discloses systems and methods for preparation of ultrapure chemicals on-site at a semiconductor manufacturing facility, so that they can be piped directly to the points of use. The disclosed systems are very compact units which can be located in the same building as a front end (or in an adjacent building), so that handling is avoided.
Hydrofluoric acid (HF) is an important process chemical in semiconductor manufacturing. It is overwhelmingly important for deglaze (i.e. removal of thin native oxides) and for oxide removal generally. It is also a component of standard wet etches for isotropic etching of silicon (e.g. "CP4A," which is 3 parts HF, 5 parts HNO.sub.3, and 3 parts acetic acid).
As noted above, the present inventors have discovered methods and systems for preparing ultra-high-purity ammonia It has now been discovered that a modification of these methods and systems can be used to prepare ultra-high-purity hydrofluoric acid.
Anhydrous HF is typically manufactured by the addition of sulfuric acid to fluorspar, CaF.sub.2. Unfortunately, many fluorspars contain arsenic, which leads to contamination of the resulting HF. Thus arsenic contamination is a dominant problem with HF purification. One source (from China) contains minimal As and is the optimal raw material for Ultra high purity HF. Low-arsenic HF, manufactured from this material and then further purified, is available from Allied Chemical in the U.S. Other impurities, in conventional systems, are contributed by the HF generation and handling system. These impurities result from degradation of these systems; these systems were designed for applications much less demanding than the semiconductor industry. These contaminants must be removed in order to achieve good semiconductor performance.
HF Vaporization
The starting material is preferably low-arsenic HF. Such acid is commercially available from Allied Chemical (manufactured in Geismar Louisiana). Such material, as used in the presently preferred embodiment, has an incoming arsenic concentration specified at &lt;1 ppb.
Optionally, however, a batch process arsenic removal and evaporation stage may be included to reduce the arsenic concentration before the Ionic Purifier column. The Ionic Purifier column is followed by the HF Supplier (HFS). In this optional embodiment, arsenic will be converted to the +5 state and held in the evaporator during distillation by the addition of an oxidant (KMnO.sub.4 or (NH.sub.4).sub.2 S.sub.2 O.sub.8) and a cation source such as KHF.sub.2 to form the salt K.sub.2 AsF.sub.7. This will be a batch process as this reaction is slow and sufficient time for completion must be allowed before the distillation takes place. This process requires contact times of approximately 1 hr at nominal temperatures. In this process the HF would be introduced into a batch process evaporator vessel and would be treated with an oxidant while stirring for a suitable reaction time.
The low-arsenic HF is then distilled in a fractionating column with reflux thus removing the bulk of the metallic impurities. Elements showing significant reduction at this step include:
______________________________________ Group 1 (I) Na, Group 2 (II) Ca, Sr, Ba, Groups 3-12 (IIIA-IIA) Cr, W, Mo, Mn, Fe, Cu, Zn Group 13 (III) Ga, Group 14 (IV) Sn, Pb, and Group 15 (VII) Sb. ______________________________________
This fractionating column acts as a series of many simple distillations; this is achieved by packing the column with a high surface area material with a counter current liquid flow thus ensuring complete equilibrium between the descending liquid and rising vapor. Only a partial condenser will be installed in this column to provide reflux and the purified gaseous HF will then be conducted to the HF Ionic Purifier (HF IP). The HF at this stage is pure by normal standards.
The HF IP will be utilized as an additional purity guarantee prior to introduction of the HF gas into the supplier systems. These elements may be present in the treatment solution or introduced in the IP to absorb sulfate carried over in the HF stream. IP testing has demonstrated significant reductions in the HF gas stream contamination for the following elements:
______________________________________ Group 2 (II) Sr, and Ba, Groups 6-12 (VIA-IIA) Cr, W, and Cu, Group 13(III) B, Group 14(IV) Pb, Sn, and Group 15 (V) Sb. ______________________________________
Many of these elements are usefull in addressing the As contamination suppression. Any carry over in the distillation column arising from their excess in the As treatment can be rectified at this step.